U.S. patent number 4,639,218 [Application Number 06/743,851] was granted by the patent office on 1987-01-27 for crystalline alumina orthodontic bracket.
This patent grant is currently assigned to Johnson & Johnson Dental Products Company. Invention is credited to Robert D. DeLuca, Robin M. F. Jones, Carlino Panzera.
United States Patent |
4,639,218 |
Jones , et al. |
January 27, 1987 |
Crystalline alumina orthodontic bracket
Abstract
There is disclosed an orthodontic bracket including a base
member for attaching to a tooth and a body member extending from
the base member. The body member includes walls that define an
archwire groove. The walls are made of single crystal alumina.
Preferably, the entire bracket is made of single crystal alumina,
and most preferably, single crystal alpha-alumina or sapphire.
Inventors: |
Jones; Robin M. F. (Titusville,
NJ), DeLuca; Robert D. (Pennington, NJ), Panzera;
Carlino (Belle Mead, NJ) |
Assignee: |
Johnson & Johnson Dental
Products Company (East Windsor, NJ)
|
Family
ID: |
24413140 |
Appl.
No.: |
06/743,851 |
Filed: |
June 12, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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602876 |
Apr 23, 1984 |
|
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707281 |
Mar 6, 1985 |
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Current U.S.
Class: |
433/8 |
Current CPC
Class: |
A61C
7/12 (20130101); Y10T 29/49568 (20150115) |
Current International
Class: |
A61C
7/12 (20060101); A61C 7/00 (20060101); A61C
007/00 () |
Field of
Search: |
;433/8,9 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Peshock; Robert
Attorney, Agent or Firm: Metz; Charles J.
Parent Case Text
This application is a continuation-in-part of our copending
applications Ser. No. 602,876, filed Apr 23, 1984, and a
continuation-in-part of Ser. No. 707,281, filed Mar. 6, 1985. Both
of said applications are now abandoned.
Claims
What is claimed is:
1. An orthodontic bracket comprising a base member for attaching to
a tooth and a body member extending from the base member, said body
member including walls defining an archwire groove, wherein said
walls comprise crystalline alumina.
2. The orthodontic bracket of claim 1 wherein said bracket is made
entirely of crystalline alumina.
3. The orthodontic bracket of claim 1 wherein said crystalline
alumina is crystalline alpha-alumina.
4. The orthodontic bracket of claim 2, wherein said crystalline
alumina is crystalline alpha-alumina.
5. The orthodontic bracket of claim 3 wherein said base member is
made of transparent plastic material and said body member is
entirely crystalline alpha-alumina.
6. The orthodontic bracket of claim 3 comprising a transparent
plastic bracket having an archwire groove lined with crystalline
alpha-alumina.
7. The orthodontic bracket of claim 1 comprising a base member
including a surface intended for adhesive contact with a tooth, and
a body member extending from said base member, wherein said surface
comprises crystalline alumina and includes an undercut portion for
enhancing the mechanical adhesion of said surface to an orthodontic
cement.
8. The orthodontic bracket of claim 7 wherein the crystalline
alumina is crystalline alpha-alumina.
9. The orthodontic bracket of claim 1 wherein the crystalline
alumina has a modulus of rupture greater than 35,000 psi.
10. The orthodontic bracket of claim 2 wherein the crystalline
alumina has a modulus of rupture greater than 35,000 psi.
11. The orthodontic bracket of claim 3 wherein the crystalline
alpha-alumina has a modulus of rupture greater than 35,000 psi.
12. The orthodontic bracket of claim 4 wherein the crystalline
alpha-alumina has a modulus of rupture greater than 35,000 psi.
13. The orthodontic bracket of claim 1 wherein aid bracket has a
rhomboidal configuration when viewed looking directly at the front
of the bracket.
14. The orthodontic bracket of claim 13 wherein both the base
member and the body member have a rhomboidal configuration.
15. The orthodontic bracket of claim 14 wherein the base member and
the body member have the same rhomboidal configuration such that
the overall rhomboidal configuration of the body member is
superimposed on the rhomboidal configuration of the base member
when the bracket is viewed looking directly at the front.
16. The orthodontic bracket of claim 13 wherein said bracket is
made entirely of crystalline alpha-alumina.
17. The orthodontic bracket of claim 14 wherein said bracket is
made entirely of crystalline alpha-alumina.
18. The orthodontic bracket of claim 15 wherein said bracket is
made entirely of crystalline alumina.
19. The orthodontic bracket of claim 14 wherein the archwire groove
is oriented essentially parallel to the top and bottom surfaces of
the bracket.
20. The orthodontic bracket of claim 15 wherein the archwire groove
is oriented essentially parallel to the top and bottom surfaces of
the bracket.
21. The orthodontic bracket of claim 19 wherein said bracket is
made entirely of crystalline alpha-alumina.
22. The orthodontic bracket of claim 20 wherein said bracket is
made entirely of crystalline alpha-alumina.
23. A crystalline alpha-alumina orthodontic bracket including a
base member for attaching to a tooth and a body member including
walls defining an archwire groove and further including two pairs
of tie wings, wherein said base member and said body member have
the same rhomboidal configuration such that the overall rhomboidal
configuration of the body member is superimposed on the rhomboidal
configuration of the base member when the bracket is viewed looking
directly at the front of the bracket.
24. The crystalline alpha-alumina orthodontic bracket of claim 23
wherein the archwire groove is oriented essentially parallel to the
top and bottom surfaces of the bracket.
Description
The invention relates to an orthodontic bracket comprising as a
load bearing member a crystalline alumina material such as
crystalline alpha-alumina.
BACKGROUND OF THE INVENTION
Orthodontic brackets attach directly to teeth and serve to transmit
corrective forces from an orthodontic archwire to the tooth to
which the bracket is attached. The requirements for an orthodontic
bracket are quite severe. First, it must have sufficient mechanical
strength to withstand the forces to which it will be subjected,
including the forces transmitted by an archwire, ligation forces,
and mastication forces. Second, it must be chemically inert in the
oral environment so that it will not corrode and will be and remain
biologically inert. The bracket must meet these rquirements, and
still remain small enough to fit on the tooth. Despite proposals
for making orthodontic brackets from many different materials, the
overwhelming majority of othodontic brackets in use today are made
of metal, usually stainless steel. Metal brackets meet all of the
essential requirements, but they have one undesirable
attribute--they are unsightly. A person undergoing orthodontic
treatment has a conspicuous amount of metal in full view on the
front surfaces of his or her teeth. And since the treatment extends
over a number of years, this unsightly appearance must be endured
for a considerable period of time.
The incentive to make brackets from less unsightly materials has
existed for many years. But recently, orthodontic treatment has
been given to increasing numbers of adults, for whom the unsightly
appearance of metal brackets is more than a mere annoyance.
Therefore, the incentive to provide more esthetic orthodontic
treatment is even greater now than it has ever been.
To avoid the unsightly appearance of metal orthodontic brackets, it
is now possible in some (but not all) cases to install the brackets
and archwire on the lingual (tongue) side of the teeth. However,
the lingual side technique usually takes much longer than the
customary buccal side technique to complete the treatment. Also,
the brackets and archwire sometimes interfere with the tongue
during speech. It has been proposed to make orthodontic brackets
out of less unsightly material, such as transparent or translucent
plastic (e.g., polycarbonate), or ceramic materials which more
closely resemble natural dentition. A problem with both plastic
materials and ceramics is that their mechanical strengths are
borderline, and bracket breakage can be a significant problem with
them. The ceramic brackets that are currently in use are rather
bulky (to overcome the physical property limitations of the
material), so they tend to be somewhat uncomfortable to the
patient. From an esthetic viewpoint, neither plastic nor ceramic
are fully satisfactory either, because plastic may discolor (from
coffee or tobacco, for example, and the color of ceramic rarely
matches natural dentition. In an effort to overcome the strength
limitations of ceramic and plastic brackets, it has been proposed
to reinforce such brackets with metal inserts or metal liners (for
the archwire groove). While this may help (although it will not
fully alleviate) the strength limitations of plastic or ceramic
brackets, such solutions bring back, to at least a limited degree,
the esthetic problem for which the plastic or ceramic bracket was
the proposed solution. Thus, to date, there is no really
satisfactory solution to the problem of unsightly metal orthodontic
brackets.
BRIEF SUMMARY OF THE INVENTION
The invention provides an orthodontic bracket comprising a base
member for attaching to a tooth and a body member extending from
the base member. The body member includes walls that define an
archwire groove, wherein said walls comprise a crystalline alumina
material such as crystalline alpha-alumina. The strength and
transparency properties of crystalline alpha-alumina and certain
other crystalline alumina materials permit the provision of
orthodontic brackets that are much more esthetic than metal
brackets, but which alleviate to a large degree the strength
limitations of plastic and ceramic brackets.
THE PRIOR ART
High alumina content, injection molded, randomly oriented,
polycrystalline ceramic orthodontic brackets are disclosed by
Reynolds in U.S. Pat. Nos. 4,216,583 and 4,322,206, and by
Wallshein in U.S. Pat. No. 4,219,617. In order to enhance adhesion
to the tooth, Reynolds mentions the possibility of providing an
undercut portion in an aperture in the tooth contacting surface of
his bracket. However, such undercut portion would have to be
machined, at prohibitive expense, since it is impossible to mold
it. The commercial version of the Reynolds bracket lacks the
undercut portion.
Plastic orthodontic brackets containing metal reinforcement and/or
metal liners for the archwire groove are disclosed by Andrews in
U.S. Pat. No. 3,930,311, Stahl in U.S. Pat. No. 3,964,165, Kurz in
U.S. Pat. No. 4,107,844, Frantz in U.S. Pat. No. 4,299,569, and
Wallshein in U.S. Pat. No. 4,302,532.
Hirabayashi et al., in U.S. Pat. No. 4,122,605, disclose a somatic
element made of single crystalline sapphire. Specific elements
disclosed include a screw type implant pin, a blade type implant
pin, a pin type implant pin, and a compression plate.
Richardson, in U.S. Pat. No. 2,045,025, discloses a method for
making orthodontic band brackets (i.e., the brackets that are
attached to tooth engaging bands) wherein a longitudinal slot is
cut in a bar of metal to form a bar that has a desired
cross-sectional configuration, followed by cutting blanks from the
bar and then machining the blanks to form the brackets.
The semi-conductor art has disclosed articles made of single
crystal alumina having a coating of silica. For instance, see
McKinnon et al., U.S. Pat. No. 3,764,507.
Hurley, in U.S. Pat. No. 3,625,740, discloses a process for
treating crystalline alpha-alumina surface with a silane to enhance
adhesion to an epoxy resin.
Daisley et al., in U.S. Pat. No. 4,415,330, disclose an orthodontic
bracket in which the tie wings have a generally rhomboidal
configuration.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an orthodontic bracket made of
crystalline alpha-alumina;
FIG. 2 is a side view of the bracket of FIG. 1;
FIG. 3 is a front view of the bracket of FIG. 1;
FIG. 4 is a top view of the bracket of FIG. 1;
FIG. 5 is a top view of a die that is used to produce a crystalline
alpha-alumina rod having a cross-sectional configuration
essentially identical to the configuration of the top of said
die;
FIG. 6 is a schematic representation of apparatus for producing a
crystalline alpha-alumina rod;
FIG. 7 is a perspective view of a crystalline alphaalumina rod
produced by the apparatus of FIG. 6;
FIG. 8 is a perspective view of a bracket blank cut from the
crystalline alpha-alumina rod of FIG. 7;
FIG. 9 is a schematic representation of apparatus for sputter
coating silica on a crystalline alpha-alumina article;
FIG. 10 is a perspective view of a plastic orthodontic bracket
having a crystalline alpha-alumina liner in the archwire
groove;
FIG. 11 is a perspective view of an orthodontic bracket having a
plastic base, with the remainder of the bracket being crystalline
alpha-alumina;
FIG. 12 is a view similar to FIG. 5, showing an alternative
configuration of the top of the die;
FIG. 13 is a perspective view of a crystalline alpha-alumina
orthodontic bracket having a keyway in the base for the purpose of
enhancing the bonding of the bracket to the tooth;
FIG. 14 is a side view of the orthodontic bracket of FIG. 13;
FIG. 15 is a perspective view of a "single-wing" orthodontic
bracket made of crystalline alpha-alumina;
FIG. 16 is a perspective view of an alternate crystalline
alpha-alumina rod that can be produced by the apparatus of FIG.
6;
FIG. 17 is a perspective view of a series of bracket blanks as they
are cut from the rod of FIG. 16;
FIG. 18 is a top plan view of the blanks of FIG. 17;
FIG. 19 is a top view of a die that is used to produce the rod of
FIG. 16;
FIG. 20 is a perspective view of an orthodontic bracket machined
from the blanks of FIGS. 17 and 18;
FIG. 21 is a front view of the bracket of FIG. 20; and
FIG. 22 is a view similar to FIG. 5, showing an alternative
configuration of the top of the die.
DETAILED DESCRIPTION OF THE INVENTION
This invention is directed to the provision of orthodontic brackets
comprising as a load bearing member certain crystalline alumina
materials, preferably crystalline alpha-alumina.
As used herein, the term "crystalline alumina" is intended to
include only essentially monocrystalline alumina, that is, alumina
comprised of a single crystal or two or more single crystals grown
together longitudinally but separated by a relatively small angle
(usually within 4.degree., determined with respect to the C-axes of
neighboring single crystals) grain boundary.
In a preferred aspect of the invention, the orthodontic bracket is
entirely crystalline alpha-alumina. Such a bracket can be produed
by first drawing a crystalline alpha-alumina rod from a melt,
wherein the rod has a predetermined cross-sectional configuration,
by slicing the rod into individual blanks, and then machining the
blanks to produce the bracket. As will be made apparent by the
discussion below, the cross-sectional configuration of the rod is
approximately the configuration of the cross-section of an
orthodontic bracket taken in a plane that is perpendicular to the
top and bottom faces of the bracket and is approximately parallel
to the two side faces of the bracket. (By "approximately parallel"
is meant not more than about 12.degree. from parallel, for reasons
that will be made clear below.) The terms "top and bottom faces"
and "side faces" refer to the top, bottom, and side surfaces,
respectively, of the bracket when looking directly at the front of
the bracket (the "front of the bracket" is the surface opposite the
tooth contacting surface) in the position the bracket would assume
when installed on a tooth with the patient in the upright position.
In the embodiments illustrated herein, the brackets have one or,
preferably, two pairs of the tie wings, and the said plane is taken
through a pair of tie wings. Thus, the said predetermined
cross-sectional configuration will include, at a minimum, a pair of
tie wings and a base.
The preferred procedure for producing a crystalline alpha-alumina
rod having a predetermined cross-sectional configuration is the EFG
(for Edge-defined, Film-fed, Growth) modification of the
Czochralski process for growing crystalline alpha-alumina. The EFG
process is described by LaBelle in "EFG--The Invention and
Application to Sapphire Growth", in Journal or Crystal Growth, 50,
pages 8-17 (September 1980). See also LaBelle, U.S. Pat. No.
3,591,348, LaBelle et al., U.S. Pat. Nos. 3,701,636 and 3,915,662,
and other patents and articles cited in the Journal of Crystal
Growth article.
FIG. 6 is a schematic representation of apparatus for producing a
crystalline alpha-alumina rod having a predetermined
cross-sectional configuration by the EFG process. The apparatus 20
includes a crucible 22 containing molten alumina 24. A die 26 made
of a suitable material such as molybdenum or iridium is positioned
such that the bottom of the die 26 is immersed in the molten
alumina 24, and the top of the die 26 is above the surface of the
melt 24. A vertical distance from the top of the melt 24 to the top
surface 28 of the die 26 of up to 50 millimeters is permissible.
(This distance is exaggerated in FIG. 6 for clarity.)
FIG. 5 shows the top surface 28 of the die 26. The top surface 28
is smooth, flat, and has the overall approximate shape of the
desired configuration of the cross-section of the crystalline
alpha-alumina rod 30 (shown in FIG. 7) from which the brackets are
made, including the configuration of a pair of tie wings, shown as
29 and 31, and the base of the bracket, shown as 33. It is
important that the sides 32 and the top surface 28 of the die 26
meet in a sharp 90.degree. angle, in order to minimize
imperfections in the surface of the growing rod 30. The die 26
contains a capillary passage 34 through which molten alumina 24 is
drawn. The melt 24 is drawn from the crucible 22 through the
capillary 34 to the top surface 28 of the die 26, where it spreads
out and completely covers the said top surface 28 with a film of
molten alumina. However, because molten alumina and molybdenum or
iridium have the appropriate wettability relationship, the molten
alumina film stops at the edge of the surface 28. Therefore,
crystalline alpha-alumina crystal grown or pulled from this film of
molten alumina assumes a cross-sectional configuration
substantially exactly the same as the configuration of the top
surface 28 of the die 26. Thus, the rod 30 (which had been started
by a seed crystal, as in the Czochralski process) pulled by a
pulling mechanism 36 from the film of molten alumina on the top
surface 28 of the die 26 will have a cross-sectional configuration
substantially identical to the configuration of the top surface 28
of the die 26. It has been found to be convenient to grow the rod
30 to a length of about two inches (about 5 centimeters) in order
to minimize any machining problems that could be caused by the
failure of the rod to grow exactly straight.
The crystal orientation of the growing rod may prove to be
important (at least economically, and perhaps also from a
performance standpoint) in the practice of the invention. In the
case of crystalline alpha-alumina, the crystal orientation can be
defined with reference to the C axis of the crystal. (The C axis is
perpendicular to the plane which contains the simplest arrangement
of atoms in the crystal unit cell. Stated another way, the C axis
is perpendicular to the plane which contains the a.sub.1 and
a.sub.2 axes.) The minimum amount of strain developed in the
growing crystal will occur if the C axis is found in a plane
perpendicular to the longitudinal axis L of the rod 30. (See FIG.
7.) This has proven to be the optimum crystal orientation in some
cases. (As is known in the art, the growing crystal will assume the
crystal orientation of the seed crystal.
Regardless of the crystal orientation of the rod 30, it is
preferred to anneal the rod 30 prior to machining so as to relieve
stresses in the crystal to minimize the chances of breakage during
machining. A typical annealing cycle would be to heat the rod 30
from room temperature up to 1850.degree. C. at an even rate for
about 12 hours, to maintain the rod 30 at 1850.degree. C. for 4 to
6 hours, and to then cool the rod 30 down to room temperature at an
even rate for 18 to 24 hours. The entire annealing cycle is
preferably carried out under an inert atmosphere such as argon.
The crystalline alpha-alumina rod 30 is cut into individual blanks
38 (FIG. 8), each of which is machined into a bracket. FIGS. 1-4
are various views of an orthodontic bracket 40 made completely of
crystalline alpha-alumina. The bracket 40 is made from the blank 38
by a series of cutting, grinding, and polishing steps, using known
techniques for machining crystalline alpha-alumina. A diamond
cutting wheel may be used to cut out the archwire groove defined by
walls 42a, 42b, 42c, 42d, 42e, 42f, and the "saddle" defined by
walls 43a, 43b, 43c of a double wing or twin bracket (such as is
shown in FIG. 1). A single wing bracket 41 is shown in FIG. 15. In
the single wing bracket 41, the archwire groove is defined by walls
42g, 42h, 42i. Edges may be beveled by grinding, and corners
rounded off by polishing.
A convenient procedure for fabricating the bracket from the
cyrstalline alpha-alumina rod 30 is the following:
The rod 30 is fastened to a rod holding fixture (not shown) with
the base surface 71 facing out. The base surface 71 is then ground
to an arcuate concavity with a diamond grinding wheel. The
resulting concave surface is shown as 74 in FIGS. 1, 2, and 8.
After the base has been ground to produce the concave surface 74,
the rod may be reversed in the fixture and the top surface 75 may
be ground to compensate for any dimensional differences arising
from the crystal growing process. This ensures a precisely
controlled base to top dimension.
The rod, with the base and top ground, may then be cut into blanks
38 (FIG. 8) with a diamond saw (not shown) by making cuts in a
plane perpendicular to the longitudinal axis L of the rod 30.
The archwire groove and the saddle are then ground with a diamond
grinding wheel. It is preferred to grind the archwire groove in two
passes. For instance, if the desired archwire groove is 20 mils
wide and 30 mils deep, the first pass will typically remove enough
material to make a groove 15 mils wide and 20 mils deep. Following
this procedure helps to minimize imperfections in the finished
bracket.
A second arcuate concavity is then ground in the base or tooth
contacting surface using a diamond grinding wheel. The thus ground
concave surface is shown as 73 in FIGS. 1 and 4. The concave
surfaces 73 and 74 are employed so that the contour of the base
more nearly matches the surface contours of a tooth.
In an alternative embodiment of the invention, the archwire groove
may be "grown" into the rod. This aspect is illustrated in FIGS.
16-21. By using a die 80 (FIG. 19) whose top surface 82 has a slot
84, a rod 86 can be grown having a longitudinal groove defined by
walls 88a, 88b, 88c in it so that, when the individual brackets are
cut from the rod 86, the brackets will already contain the archwire
groove, as defined by the walls 88a, 88b, and 88c. By so doing, one
step (i.e., the grinding of the archwire groove) in the procedure
for producing the bracket can be eliminated, at a significant cost
saving.
Because the dimensions of the cross-section of the archwire groove
are quite small (e.g., 20 by 30 mils), it may be difficult to grow
the archwire groove in the rod because the surface tension of the
molten alumina may tend to close up the groove. Therefore, the
archwire groove may also be ground in the rod to produce the
grooved rod shown in FIG. 16. This is also a cost saving procedure
because it is easier to handle the whole rod in the grinding
operation than the individual blanks cut from the rod. It is
probable that, even where the groove is grown in the rod, some
grinding will be necessary to finish the groove to the desired
dimensions.
One difference in the procedure for making the brackets in
accordance with this alternative embodiment of the invention,
whether the archwire groove is grown in the rod or ground in it, is
that the bracket blanks 92 that are cut from the rod 86 are cut at
a slight angle. Thus, instead of making the cuts in the rod 86 in a
plane normal or perpendicular to the longitudinal axis L of the
rod, the cuts are made in the following manner:
Holding the rod 86 in position with the longitudinal axis L in a
horizontal plane and the face having the longitudinal groove on
top, each cut is made in a vertical plane that is angled slightly
(e.g., up to about 12.degree.) at an angle .alpha. from the
vertical plane that is perpendicular to the longitudinal axis L of
the rod 86. This is best seen in FIG. 18.
The saddles and the second base concavities 73 can be machined in
the bracket prior to cutting the individual brackets from the rod
86. This is preferred because it is easier to handle the rod 86
than the individual bracket blanks 92. The machining of the saddles
can then be done with a diamond grinding wheel by feeding the rod
stepwise to the wheel, and the two base concavities can be ground
in a similar stepwise manner with a diamond grinding wheel whose
grinding edge is rounded or radiused to the appropriate degree so
that the two concavities can be ground at the same time.
FIGS. 20 and 21 are perspective and front views, respectively, of a
bracket 94 made from the blank 92.
As can be seen most clearly in FIG. 21, the brackets 94 produced in
accordance with this alternative embodiment of the invention have
rhomboidal configuration when viewed looking directly at the front
of the bracket. Referring again to FIG. 21, both the body portion
of the bracket (which includes the tie wings 96, 98, 100, 102) and
the base 104 have a rhomboidal configuration. Preferably, the body
portion and the base have the same rhomboidal configuration with
the overall rhomboidal configuration of the body portion being
superimposed on that of the base when the bracket is viewed looking
directly at the front, as in FIG. 21. It is noted further that the
archwire groove is oriented such that it is essentially parallel to
the top and bottom surfaces of the bracket 94. That is, the
longitudinal axis Lg of the archwire groove is parallel to the
lines Lt and Lb that are drawn through the top and bottom edges,
respectively, of the bracket 94 when viewed looking directly at the
front.
To reduce the amount of grinding to a minimum, the rod may be grown
such that the top and bottom faces are parallel to each other. This
can be done using a die 110 configured as shown in FIG. 22, wherein
the edge shown as 112 is parallel to the edge shown as 114. The rod
(not shown) grown using this die 110 need only have the top and
bottom surfaces lapped simultaneously, to insure uniform thickness,
prior to the performance of the other machining operations, as
described above.
After machining, the brackets are preferably annealed under the
conditions disclosed above for drawn rods. Then, the brackets are
preferably polished to smooth off contours and to remove any
surface imperfections which could encourage propagation of cracks.
A flux polishing procedure is recommended, in which the flux is
partially saturated with alumina so that the removal of alumina
from the surface of the bracket will proceed at a controllable
rate. One preferred flux is composed of 51.2 per cent LiBO.sub.2,
12.8 per cent Li.sub.2 B.sub.4 O.sub.7, 16 per cent Al.sub.2
O.sub.3, and 20 per cent LiF (the percentages are by weight). The
machined brackets are immersed in molten flux at 850.degree. to
900.degree. C. for a few minutes, e.g., from about four to about
thirty minutes, and then removed. After cooling, the brackets can
be immersed in aqueous hydrofluoric acid to remove any flux
adhering to the surfaces of the brackets.
Other processes for polishing the surface of crystalline
alpha-alumina objects are known, and may be used if desired. Such
other processes are disclosed, for example, by Noble, in U.S. Pat.
No. 4,339,300, and Manasevit, in U.S. Pat. No. 3,546,036.
In alternative embodiments of the invention, the most critical load
bearing portions of the bracket are made of a crystalline alumina
material, while the remainder is made of another transparent
material, such as polycarbonate or polysulfone plastic, that is
less expensive, easier to work, and easier to bond to the tooth.
FIG. 10 shows one such alternative embodiment, wherein the bracket
44 is made predominantly of transparent plastic 46 (e.g.,
polycarbonate), but wherein the archwire groove has crystalline
alumina liners 48a, 48b cemented therein. In another embodiment,
shown in FIG. 11, the bracket 50 has a transparent plastic base 52
(as the tooth contacting portion) cemented to a crystalline alumina
body 54. In both of these alternative embodiments, the crystalline
alumina portions can be made by a modification of the method
described above, starting with a crystalline alumina rod of
appropriate shape made by the EFG process.
Bonding a crystalline alumina bracket to the tooth (or to a plastic
base or to any other substrate) must be done with care. Many of the
ordinary orthodontic cements (which are usually acrylic resins)
will not adhere well to crystalline alumina without taking steps to
enhance the adhesion. One means of enhancing the adhesion of a
crystalline alumina bracket to the tooth is illustrated in FIGS. 13
and 14, in which a bracket 56 is shown that has an undercut or
keyway 58 in the bottom or tooth-contacting surface of the bracket
56. Orthodontic cement filling the keyway 58 will have enhanced
mechanical adhesion to the bracket 56 because of the undercut
portion. This bracket 56 can be made by a method analogous to that
described above, starting with the EFG process using a molybdenum
die 60 having a top surface 28a shaped as shown in FIG. 12. The
undercuts 58 can also serve as slots for the insertion of pliers or
the like to facilitate removal of the brackets at the conclusion of
the orthodontic treatment.
Another means of enhancing the adhesion of cements such as acrylic
resins to a crystalline alumina bracket is to alter the surface of
the crystalline alumina in such a way as to increase the strength
of the adhesive bond between the crystalline alumina and the
cement. It is known, for instance, that a wide variety of silicone
coupling agents can be used to enhance the adhesive force between
siliceous substrates and a wide variety of thermosetting plastics.
This technology may be utilized by coating the crystalline alumina
surface that is to be in contact with the cement with a thin
coating (usually thinner than about 10,000 angstroms, and
preferably, up to about 1,000 angstroms) of a siliceous material
such as silica, and then using silicone or silane coupling agents
to enhacne the bond of that surface to the cement, in a manner
analogous to that which is presently known. Examples of means for
coating the crystalline alumina surface with a siliceous material
are cathode sputtering, plasma deposition, and electron beam
evaporation, all of which are known techniques, especially in the
semi-conductor arts. FIG. 9 is a schematic representation of
apparatus suitable for sputter coating silica onto the surface of a
crystalline alumina orthodontic bracket. The apparatus, shown
generally as 62, includes a sputtering chamber 64 (which is vacuum
tight), a target 66, in this case silicon metal, which is brought
to cathode potential, an RF or DC power supply 68, and a plate 70
for holding the cleaned and dried substrate 72 to be coated, in
which the plate 70 is brought to anode potential. A source of
oxygen (not shown) introduces oxygen into the chamber 64 so that
the silicon metal 66 will be converted to silicon dioxide on the
substrate 72. Reactive sputtering, such as is briefly outlined
here, is known. For instance, see "The Basics of Sputtering",
printed in December 1980 by Materials Research Corporation,
Orangeburg, N.Y. 10962.
The crystalline alumina bracket having its base or tooth-contacting
surface sputter coated with silica or other siliceous material such
as a glass, has excellent affinity for silicone coupling agents
such as A-174 (gamma-methacryloxypropyltrimethoxysilane), and by
using such coupling agents the adhesion of the bracket to acrylic
orthodontic cements is enhanced. Before applying the coupling
agent, the silica-coated bracket should be heated in air for about
1 hour at 350.degree. C. to convert the silica surface to a form
that has a greater affinity for the coupling agent. For a fuller
description of the use of a thin siliceous coating on the surface
of crystalline alumina to enhance the adhesive bond to cements, see
U.S. patent application Ser. No. 602,874, for "Crystalline Alumina
Composites", filed on Apr. 23, 1984, and assigned to the same
assignee as this application.
Another method for enhancing the affinity of the crystalline
alpha-alumina bracket to silicone coupling agents is to heat the
brackets to remove adsorbed water, and then treat the bracket with
a dilute solution (e.g., a 2 to 2.5 weight per cent solution in
toluene/propylene glycol monomethyl ether) of a silane coupling
agent such as A-174. A heat treatment in air at 350.degree. C.
overnight (about 16 hours) has been found to be satisfactory.
Alternatively, a short (about 1/2 hour) treatment in vacuum at
110.degree. C. followed by heating in air at 350.degree. C. for
about three hours may be used. In both cases, the heat treated
crystalline alumina bracket should be protected from moisture prior
to the silane treatment. After treatment with the silane, a
post-cure at, e.g., 110.degree. C. for about 1 to 3 hours, is
recommended to develop the optimum bonding strength.
The orthodontic brackets of the invention have enhanced esthetics
because of the transparency of crystalline alumina. For instance,
the transparency of crystalline alpha-alumina is such that a total
of up to 98.5 per cent of light in the visible range is transmitted
through it, as determined by the integrating sphere method.
The yield strength of the steel that is used to make orthodontic
brackets is typically about 35,000 to 40,000 psi. The modulus of
rupture of crystalline alphalumina used in the invention is at
least 35,000 to 40,000, and is often as high as about 100,000 psi.
Therefore, the effective strength of the brackets of the invention
is at least as high as that of the usual steel bracket and often
much higher, but with significantly enhanced esthetics. (The
modulus of rupture is determined at 25.degree. C. by the test
procedure of ATM C-674.)
The invention has been described most particularly with respect to
the use of crystalline alpha-alumina (sapphire) as the material
from which the subject orthodontic brackets are made. However,
other crystalline alumina materials can be used in the invention.
The limiting requirements are adequate modulus of rupture (i.e., at
least as great as the yield strength of the steel that is currently
used for most orthodontic brackets), and sufficient transparency so
that the natural tooth color can be seen through the bracket. Other
crystalline alumina materials that can be used include yttrium
aluminum garnet, magnesium aluminum spinel, and alpha-alumina in
which a small percentage of the aluminum atoms has been replaced
with other elements to impart color and/or fluorescence to the
crystal. For instance, fluorescence can be imparted to the crystal
by the addition of small amounts (e.g., less than 1 mole per cent)
of terbium oxide or cerium oxide to the aluminum oxide.
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